Micromechanical sensor and methods for producing a micromechanical sensor and a micromechanical sensor element

ABSTRACT

A method produces a micromechanical sensor element having a first electrode and a second electrode, wherein electrode wall surfaces of the first and the second electrodes are situated opposite one another in a first direction and form a capacitance, wherein one of the first electrode or the second electrode is movable in a second direction, in response to a variable to be detected, and a second one of the first electrode and the second electrode is fixed. The method includes producing a cavity in a semiconductor substrate, the cavity being closed by a doped semiconductor layer; producing the first and the second electrodes in the semiconductor layer, including modifying the electrode wall surface of the first electrode in order to have a smaller extent in the second direction than the electrode wall surface of the second electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.16/018,169 filed Jun. 26, 2018, which claims the benefit of GermanPatent Application No. 10 2017 211 080.5 filed Jun. 29, 2017, which areincorporated by reference as if fully set forth.

FIELD

The present disclosure relates to micromechanical sensors,micromechanical sensor elements, and methods for producing same. Inparticular, the present disclosure relates to sensors and sensorelements having at least one electrode which is movable in response to avariable to be detected, for example, a pressure or an acceleration.

BACKGROUND

Piezoresistive sensors or capacitive sensors can be used foracceleration detection or pressure detection.

In the case of piezoresistive sensors, beams bend in response to apressure or an acceleration, as a result of which an electricalresistance of the beams changes. The corresponding change in resistancecan be detected in order to deduce on the basis thereof the variable tobe detected, for example the pressure or the acceleration.

Capacitive sensor elements have capacitances, at least one electrode ofwhich is movable. A movement of the movable electrode can be broughtabout by a variable to be detected, for example a pressure or anacceleration, which in turn results in a change in capacitance that canbe detected in order to deduce the variable to be detected.

For signal detection, corresponding sensor elements can beinterconnected in Wheatstone bridges. Wafer technologies can be used toproduce movable structures of the sensor elements by micromachining.Wafer bonding can be used to achieve an encapsulation of thecorresponding structures on the front side and the rear side and hencesufficient robustness.

Capacitive acceleration sensors can be designed to detect anacceleration out of plane. In this case, a movable test mass can beanchored with a stationary substrate by means of an anchor, whereinelectrodes are formed both on the test mass and on the stationarysubstrate, said electrodes forming two capacitors on two sides of theanchor. As a reaction to an acceleration out of plane, the test massinclines relative to its rotation axis, which changes the capacitancesof the two capacitors. This imbalance can be detected in order to detectthe acceleration out of plane.

SUMMARY

Micromechanical sensors and micromechanical sensor elements havingimproved characteristics would be desirable.

Examples of the present disclosure provide a micromechanical sensorincluding a first and a second capacitive sensor element each having afirst and a second electrode, wherein electrode wall surfaces of thefirst electrode and the second electrode are situated opposite oneanother in a first direction and form a capacitance, wherein the firstelectrodes are movable in a second direction, which is different thanthe first direction, in response to a variable to be detected, and thesecond electrodes are stationary. The electrode wall surface of thefirst electrode of the first sensor element has a smaller extent in thesecond direction than the opposite electrode wall surface of the secondelectrode of the first sensor element. The electrode wall surface of thesecond electrode of the second sensor element has a smaller extent inthe second direction than the opposite electrode wall surface of thefirst electrode of the second sensor element.

Examples of the present disclosure provide a method for producing amicromechanical sensor, wherein a first and a second capacitive sensorelement are produced, each having a first and a second electrode,wherein electrode wall surface of the first electrode and the secondelectrode are situated opposite one another in a first direction andform a capacitance, wherein the first electrodes are movable in a seconddirection, which is different than the first direction, in response to avariable to be detected, and the second electrodes are stationary. Inthis case, the electrode wall surface of the first electrode of thefirst sensor element is produced with an extent in the second directionwhich is smaller than an extent of the opposite electrode wall surfaceof the second electrode of the first sensor element in the seconddirection. The electrode wall surface of the second electrode of thesecond sensor element is produced with an extent in the second directionwhich is smaller than an extent of the opposite electrode wall surfaceof the first electrode of the second element in the second direction.

Examples of the present disclosure provide a method for producing amicromechanical sensor element having a first and a second electrode,wherein electrode wall surfaces of the first and second electrodes aresituated opposite one another in a first direction and form acapacitance, wherein the first electrode is movable in a seconddirection, which is different than the first direction, in response to avariable to be detected, and the second electrode is stationary. In thiscase, a cavity is produced in a semiconductor substrate, said cavitybeing closed by a doped semiconductor layer. Both electrodes of thecapacitive sensor element are produced in the semiconductor layer,wherein the electrode wall surface of one of the two electrodes ismodified in order to have a smaller or larger extent in the seconddirection than the opposite electrode wall surface of the other of thetwo electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the disclosure are described below with reference to theaccompanying drawings, in which:

FIGS. 1A and 1B show schematic illustrations of one example of amicromechanical sensor.

FIG. 2 shows schematic illustrations of examples of capacitive sensorelements in which electrodes are modified differently.

FIG. 3 shows schematic illustrations of simulation results of theexemplary structures of capacitive sensor elements shown in FIG. 2.

FIG. 4 shows an illustration of one example of a half-bridge evaluationcircuit.

FIG. 5 shows an illustration of one example of a full-bridge evaluationcircuit.

FIGS. 6A and 6B show schematic sectional views of examples of first andsecond capacitive sensor elements having dielectrically isolatedelectrodes.

FIGS. 7A and 7B show schematic sectional views of examples of first andsecond capacitive sensor elements having a movable electrode suspendedfrom a substrate.

FIGS. 8A and 8B show schematic sectional views of examples of first andsecond capacitive sensor elements having PN isolation.

FIGS. 9A and 9B show schematic plan views of examples of substrates of afirst and a second capacitive sensor element in accordance with FIGS. 6Aand 6B.

FIGS. 9C and 9D show schematic plan views of examples of substrates of afirst and a second capacitive sensor element in accordance with FIGS. 7Aand 7B.

FIGS. 9E and 9F show schematic plan views of examples of substrates of afirst and a second capacitive sensor element in accordance with FIGS. 8Aand 8B.

FIG. 10 shows a schematic sectional view of one example of a capacitivesensor element having an electrode modified by an insulator.

FIG. 11 shows a schematic sectional view of one example of a capacitivesensor element having an electrode modified by a counterdoping.

FIG. 12 shows a schematic illustration of one example of a capacitivesensor element having an electrode modified by a cutout.

FIGS. 13A to 13C show schematic cross-sectional views for elucidatingone example of a method for forming a cavity in a substrate.

FIG. 14 shows a schematic sectional view for elucidating one example ofa method for producing contacts through an oxide layer.

FIGS. 15A to 15I show schematic sectional views for elucidating oneexample of a method for producing a capacitive sensor element having anelectrode modified by an insulator.

FIGS. 16A to 16I show schematic sectional views for elucidating oneexample of a method for producing a capacitive sensor element having anelectrode modified by a counterimplantation.

FIGS. 17A to 17C show schematic sectional views for elucidating oneexample of a method for producing a capacitive sensor element having anelectrode modified by a cutout.

FIGS. 18A and 18B show schematic plan views of examples of a firstsensor element having a modified stationary electrode and of a secondsensor element having a modified movable electrode.

FIG. 19 shows a schematic plan view of one example of a micromechanicalsensor in which a first and a second capacitive sensor element have acommon movable element.

FIGS. 20A and 20B show schematic plan views for elucidating one exampleof a capacitive sensor element having interdigital electrodes.

DETAILED DESCRIPTION

Examples of the present disclosure are described below in detail andusing the accompanying drawings. It should be pointed out that identicalelements or elements having the same functionality are provided withidentical or similar reference signs, a repeated description of elementsthat are provided with the same or similar reference signs typicallybeing omitted. Descriptions of elements having identical or similarreference signs are mutually interchangeable. In the followingdescription, many details are described in order to provide a morethorough explanation of examples of the disclosure. However, it isevident to those skilled in the art that other examples can beimplemented without these specific details. Features of the variousexamples described can be combined with one another, unless features ofa corresponding combination are mutually exclusive or such a combinationis expressly precluded.

FIG. 1A schematically shows a sectional view of a first capacitivesensor element 2 and a second capacitive sensor element 4. The firstcapacitive sensor element 2 has a first electrode 10 and a secondelectrode 12. The second capacitive sensor element 4 has a firstelectrode 14 and a second electrode 16. Electrode wall surfaces 10 a, 12a of the first electrode 10 and of the second electrode 12 are situatedopposite one another in a first direction R1 and form a capacitance.Electrode wall surfaces 14 a, 16 a of the first electrode 14 and of thesecond electrode 16 are likewise situated opposite one another in thefirst direction R1 and form a capacitance. The first electrodes 10 and14 are movable in a second direction R2, which is different than thefirst direction R1. The second electrodes 12 and 16 are stationary.

In examples, the first electrodes 10 and 14 can be mounted movably on acarrier and the second electrodes 12 and 16 can be mounted in astationary manner on the carrier. The carrier can be a substrate, forexample. The electrodes can be formed for example on or in a substrate,wherein the first direction R1 is parallel to a substrate plane, and thesecond direction is perpendicular to the substrate plane. In this case,the substrate plane can be a plane that is parallel to the main surfacesof the substrate. In this case, a substrate can be understood to mean abody having two mutually opposite main surfaces connected by respectiveside surfaces. In this case, the main surfaces are the surfaces of thesubstrate which have a larger area than the side surfaces. In examples,the substrate can be a semiconductor substrate, such as e.g. a siliconsubstrate. The movable electrodes can be mounted in a resilient manneron the substrate and can be movable, i.e. deflectable, from a rest stateinto a deflected state in response to a force. Once the force actionends, the movable electrodes can return to the rest state again byvirtue of the resilient mounting. The movable electrodes can thus bedeflected in response to an out of plane force, that is to say a forcethat acts perpendicularly to the substrate plane.

In examples of the present disclosure, the electrodes having themutually opposite electrode wall surfaces can be implemented in anydesired manner. In examples, the electrodes can be formed by dopedsemiconductor regions of a substrate. In examples, the electrodes can beformed in a semiconductor substrate, such as e.g. a silicon substrate,which has a doping of >5×10¹⁷. In other examples, the electrodes can beformed from some other conductive material, for example metal. Inexamples, the electrodes can be formed by conductive coatings on aninsulating material.

The electrode wall surfaces 10 a, 12 a, 14 a and 16 a can be planar andarranged in a plane that is perpendicular to the first direction R1 andparallel to the second direction R2, that is to say in a plane that isdefined by the second direction R2 and a third direction R3, which isperpendicular to the first direction R1 and the second direction R2. Inexamples, said plane in which the electrode wall surfaces are arrangedcan be perpendicular to the substrate plane. The extent of the electrodewall surfaces in the second direction can be constant, as can be seen inFIG. 1B.

The electrode wall surface 10 a of the first electrode 10 of the firstsensor element 2 has a smaller extent in the second direction R2 thanthe opposite electrode wall surface 12 a of the second electrode 12 ofthe first sensor element 2. The electrode wall surface 16 a of thesecond electrode 16 of the second sensor element 4 has a smaller extentin the second direction R2 than the opposite electrode wall surface 14 aof the first electrode 14 of the second sensor element 4.

As is indicated in FIG. 1A, the electrodes 10, 12, 14 and 16 can beelectrically connected to an evaluation circuit 20. The evaluationcircuit 20 can be designed to detect the capacitances of the firstcapacitive sensor element 2 and of the second capacitive sensor element4. Using the detected capacitances, it is then possible to deduce avariable to be detected which brings about a movement of the movableelectrodes 10 and 14 in the direction R2 or counter to the direction R2.Such a movement of the movable electrodes 10 and 14 relative to thestationary electrodes 12 and 16 brings about a change in capacitance ofat least one of the capacitive sensor elements, which is in turndetectable.

In examples, the electrode wall surface 10 a of the first electrode 10of the first sensor element 2 can have the same extent in the seconddirection R2 as the electrode wall surface 16 a of the second electrode16 of the second sensor element 4. The electrode wall surface 12 a ofthe second electrode of the first sensor element 2 can have the sameextent in the second direction R2 as the electrode wall surface 14 a ofthe first electrode 14 of the second sensor element 4.

In examples, the mutually opposite electrode wall surfaces 10 a and 12 aof the first and second electrodes 10 and 12 of the first sensor element2 can have the same extent in the direction R3. In examples, themutually opposite electrode wall surfaces 14 a and 16 a of the first andsecond electrodes 14 and 16 of the second sensor element 4 can have thesame extent in the direction R3. In examples, all electrodes can havethe same extent in the direction R3.

In examples, first ends of the electrode wall surfaces of the electrodes10, 12, 14 and 16 can extend in the direction R2 as far as a commonfirst plane. Second ends of the electrode wall surfaces 10 a and 16 asituated opposite the first ends can extend as far as a second plane,which is parallel to the first plane. Second ends of the electrode wallsurfaces 12 a and 14 a situated opposite the first ends can extend asfar as a third plane, which is parallel to the first and second planes,and which is spaced apart from the first plane further than the secondplane.

The first electrodes 10 and 14 are movable in the direction R2 that isdifferent than the direction R1. The direction R2 can be perpendicularto the direction R1. The direction R1 can be parallel to the substrateplane and the direction R2 can be perpendicular to the substrate plane.In examples, the movement of the electrodes can have components in otherdirections besides the principal component in the second direction R2,as long as the component of the movement in the second direction is thelargest component.

Examples of the present disclosure use a capacitive out-of-planedetection principle with an electrode system of mutually laterallyopposite conductor structures, i.e. conductive electrodes. In this case,the expression “out of plane” can relate to the substrate plane, and theexpressions “lateral(ly)” and “vertical(ly)” can likewise relate to thearrangement relative to the substrate plane. Under the action of a forcethat is intended to be detected, the electrodes of a capacitive sensorelement can experience a displacement of the vertical position relativeto one another, which leads to a corresponding change in the capacitancebetween the electrodes of the capacitive sensor element. It has beenrecognized that the general characteristic of such a capacitive sensorelement can be more or less nonlinear or can even have a maximum in aninput range of interest, which can lead to ambivalent results.

Examples of the present disclosure provide a micromechanical sensor thatuses two capacitive sensor elements in order to obtain an output signalhaving a more linear characteristic than a single capacitive sensorelement. Examples of the present disclosure are geared toward attainingan approximately linear differential change in capacitance with theinput signal, i.e. [ΔC1(signal)−ΔC2(signal)]/Δsignal=constant. In thiscase, C1(signal) represents the capacitance of the first capacitivesensor element 2, said capacitance being dependent on the signal to bedetected, and C2(signal) represents the capacitance of the secondcapacitive sensor element 4, said capacitance being dependent on thesignal to be detected. The symbol Δ represents the change in therespective variable.

For this purpose, in examples, in one of the capacitive sensor elements,the movable electrode can have an electrode wall surface which has asmaller extent in the second direction than the opposite electrode wallsurface of the stationary electrode and, in the other capacitive sensorelement, the stationary electrode can have an electrode wall surfacewhich has a smaller extent in the second direction than the electrodewall surface of the opposite movable electrode. The capacitive elementscan thus be suitably modified in order to obtain a more linear outputsignal dependent on the input signal.

By means of a modification of a respective electrode in each sensorelement in order to bring about different extents of mutually oppositeelectrode surfaces in the direction of movement, it is thus possible tointroduce an asymmetry into an otherwise symmetrical vertical, geometricand electrical profile. In one sensor element the modified electrode ismovable, and in the other sensor element the modified electrode isstationary.

This behavior was shown on the basis of a simulation. FIG. 2schematically shows capacitive sensor elements, wherein the middlecolumn shows the respective sensor element in the rest state, i.e. for adeflection of 0. The left-hand column shows the respective sensorelement for a deflection −x₁ of the movable electrode counter to thedirection R2, while the right-hand column shows the respective sensorelement for a deflection x₁ of the movable electrode in the directionR2.

The sensor elements shown in FIG. 2 each have a stationary electrode 30and a movable electrode 32, between which a capacitance is formed.Furthermore, schematic sections of a substrate 34 and of a covering 36of the respective sensor element are shown in FIG. 2. The first rowshows in each case a capacitive sensor element in which the oppositeelectrode wall surfaces of the first and second electrodes 30 and 32have an identical extent in the second direction. The electrodes arethus symmetrical in this case. In the middle row in FIG. 2, thestationary electrode 30 has a smaller extent in the second direction,which is indicated by an insulating region 38. In examples, thestationary electrode (Efix) can be modified for this purpose. In thebottom row in FIG. 2, the movable electrode 32 has a smaller extent inthe second direction R2, which is indicated by an insulating region 40.In examples, the movable electrode (Emov) can be modified for thispurpose.

As is shown in the left-hand column and the right-hand column in FIG. 2,a force acting on the movable electrode 32 brings about a deflection −x₁counter to the second direction R2 or a deflection x₁ in the seconddirection R2. In examples, such a force can be brought about by avariable to be detected, such as, for example, a pressure or anacceleration. The results of a two-dimensional simulation of thecapacitance between the mutually opposite electrodes of the capacitivesensor elements from FIG. 2 is shown in FIG. 3. The upper region of FIG.3 here illustrates in each case the capacitance of a single capacitiveelement, wherein curve 41 shows the characteristic of a single,non-modified element, curve 43 shows the characteristic of a singleelement in which the movable electrode is modified, and curve 45 showsthe characteristic of a single element in which the stationary electrodewas modified. The capacitance is plotted here in each case against thedeflection of the movable electrode. The lower part of FIG. 3 shows thedifference signal of two capacitive sensor elements. Curve 47 shows thedifference between two symmetrical sensor elements, such that thedifference output is 0. Curve 49 shows the difference between a firstcapacitive sensor element, in which the movable electrode is modified,and a second sensor element, in which the stationary electrode ismodified. Curve 49 shows that a substantially linear differential changein capacitance can be attained. FIG. 3 furthermore shows that it ispossible to attain a linear characteristic over a large range andspecifically in both deflection directions proceeding from theforce-free rest state.

In examples of the present disclosure, a micromechanical sensor has anevaluation circuit configured to output an output signal which isproportional to the difference between the capacitances of the first andsecond capacitive sensor elements.

In examples, the evaluation circuit can have a half-bridge circuit,wherein the first and second sensor element are interconnected in thehalf-bridge circuit. One example of such a half-bridge circuit is shownin FIG. 4, wherein C2 represents the capacitance of a capacitive sensorelement in which the movable electrode is modified, while C1 representsthe capacitance of a sensor element in which the stationary sensorelectrode is modified. v1 and v2 represent supply voltages. c_feedbackrepresents a feedback capacitor for a differential amplifier 50. Theelements are interconnected in the manner shown in FIG. 4 in order toobtain the output signal v_out at the output of the differentialamplifier 50. FIG. 4 furthermore shows how the output signal can becalculated from the indicated variables in the case of the half-bridgecircuit shown.

In examples of the present disclosure, the micromechanical sensor canhave two corresponding first capacitive sensor elements and twocorresponding second capacitive sensor elements, wherein the evaluationcircuit can have a full-bridge circuit, and wherein the first and secondsensor elements can be interconnected in the full-bridge circuit. Oneexample of such a full-bridge circuit is shown in FIG. 5. Two capacitivesensor elements having a modified movable electrode, which haverespective capacitances C2, and two capacitive sensor elements having amodified stationary electrode, which have respective capacitances C1,are connected to the inputs of a differential amplifier 52 having adifferential output v_out in the manner shown in FIG. 5. Two feedbackcapacitors c_feedback are interconnected in the manner shown. A signalsource 54 supplies a rectangular input voltage v_in. FIG. 5 furthermoreindicates how the output signal v_out can be calculated from the givenvariables in the case of the full-bridge circuit shown. Additionalcommon-mode rejection can be achieved through the use of a full-bridgeconfiguration.

Referring to FIGS. 6 to 9, examples in which the electrodes are formedby doped semiconductor regions of a substrate are described below.

FIGS. 6A and 6B show schematic cross-sectional views of capacitivesensor elements which are implemented with dielectrically isolatedelectrodes, FIG. 9A shows a schematic plan view of the substrate of thesensor element from FIG. 6A, and FIG. 9B shows a schematic plan view ofthe substrate of the sensor element from FIG. 6B.

In the sensor element shown in FIG. 6A, a movable electrode 14 and astationary electrode 16 are structured in a semiconductor substrate 100.The semiconductor substrate 100 can be at least partly doped, such thatthe electrodes 14 and 16 are conductive. The substrate 100 has a buriedcavity 102 and trenches 104, which define the electrodes 14 and 16. Inthis case, FIG. 9A shows a possible course of the trenches defining theelectrodes 14 and 16. An insulating layer 110 is provided on a mainsurface of the substrate 100. The movable electrode 14 and thestationary electrode 16 are applied to the insulating layer 110.Consequently, the movable electrode 14 and the stationary electrode 16are insulated from one another by the trenches 104, the buried cavity102 and the insulating layer 110. The insulating layer 110 can consistof a dielectric such as e.g. an oxide. Furthermore, the capacitivesensor element has a contact 106 for the movable electrode 14 and acontact 108 for the stationary electrode 16.

The stationary electrode 16 is modified in such a way that an electrodewall surface 16 a thereof situated opposite an electrode wall surface 14a of the movable electrode 14 has a smaller extent perpendicular to thesubstrate plane than the electrode wall surface 14 a. For this purpose,a section 111 of the electrode 16 is replaced by an insulating material,for example an oxide. The movable electrode is movable in response to anout of plane force, as is indicated by an arrow 112 in FIG. 6A. FIG. 6Athus shows one example of a second capacitive sensor element 4 in whichthe stationary electrode has a smaller extent in the direction ofmovement.

The trenches 104 can be designed in such a way that an elongatedeflectable electrode 14 is produced, which is clamped at one end 114and whose end at a distance from this clamped end is movable.

FIG. 6B shows a corresponding capacitive sensor element in which asubstrate 100 is structured by a buried cavity 102 and trenches 104 inorder to implement a movable electrode 10 and a stationary electrode 12.The movable electrode 10 is once again designed as an elongate electrodewhich is clamped at a first end 114 by means of the insulating layer 110and whose second end is movable vertically with respect to the substrateplane, as is once again indicated by an arrow 112. In the capacitivesensor element shown in FIG. 6B, an electrode wall surface 10 a of themovable electrode 10 has a smaller extent in the direction of movementthan an electrode wall surface 12 a of the stationary electrode 12. Inthe example shown, this is implemented by a part of the movableelectrode 10 being replaced by an insulating material 116. Thecapacitive sensor element shown in FIG. 6B thus represents one exampleof a first capacitive sensor element 2 in which the movable electrodehas a smaller extent in the direction of movement.

Since, in the sensor elements shown in FIGS. 6A and 6B, the movableelectrode is completely surrounded by the trench 104, the stationaryelectrode can be mounted at the semiconductor substrate and need not becompletely surrounded by the trench 104.

FIGS. 7A and 7B show schematic sectional views of examples of a secondsensor element 4 and a first sensor element 2, which differ from thecapacitive sensor elements shown in FIGS. 6A and 6B with regard to thesuspension of the movable electrodes, and so the following descriptionis directed in particular to these differences. FIGS. 9C and 9D showschematic plan views of the substrates 100 of the capacitive sensorelements shown in FIGS. 7A and 7B, wherein the course of the trenches inthe region of the clamped end of the movable electrodes 10 and 14 isdifferent than the course of the trenches in the examples shown in FIGS.6A and 6B. To put it more precisely, the trenches 104 in the examplesshown in FIGS. 7A and 7B do not isolate the movable electrodes 10 and 14from the substrate 100, with the result that the movable electrodes 10and 14 are suspended from the substrate 100 at a first end 120. For therest, the construction corresponds to that of the examples describedwith reference to FIGS. 6A and 6B, and so a detailed description can beomitted.

FIGS. 8A and 8B show schematic sectional views of examples of a secondcapacitive sensor element 4 and of a first capacitive sensor element 2.FIGS. 9E and 9F show schematic plan views of the substrate of theexamples shown in FIGS. 8A and 8B. The examples shown in FIGS. 8A and 8Bdiffer from the examples shown in FIGS. 7A and 7B in the type ofisolation of the movable electrodes 10 and 14 from the stationaryelectrodes 12 and 16. The substrate 100 comprises two layers 130 and 132having a different doping type. By way of example, the layer 130 canhave an n-type doping, and the layer 132 can have a p-type doping. As isshown, the buried cavity 102 can be formed in both layers 130 and 132.The electrodes are structured in the upper semiconductor layer 130. Themovable electrodes 10 and 14 are once again suspended from the substrateat a first end 120, while a second end at a distance therefrom is onceagain movable, as is indicated by an arrow 112. In the example shown inFIGS. 8A and 8B, the stationary electrodes 12 and 16 are isolated notonly by the trenches 104 but moreover by the pn junction between thesemiconductor layers 130 and 132 having different doping types. In thisrespect, in the examples shown in FIGS. 8A and 8B, the stationaryelectrode need not be completely surrounded by a buried cavity andtrenches, but rather can be mounted at a part of the semiconductor layer130.

Referring to FIGS. 10 to 12, three examples of how a correspondingmodification of the respective electrode which has the smaller dimensionin the direction of movement can be achieved are described below.Referring to FIGS. 13 to 17, examples of methods for producingcorresponding modifications are then described. It should be pointed outat this juncture that the corresponding figures are purely schematic,wherein in each case one of the electrodes can be designed as a movableelectrode and one of the electrodes as a stationary electrode. By way ofexample, these electrodes can therefore be implemented in the manner asdescribed above with reference to FIGS. 6 to 9.

In examples of the present disclosure, the first and second electrodescan be delimited on one side in each case by a cavity in a substrate. Inexamples, said cavity can be produced by means of a so-called Veneziamethod, as described below with reference to FIGS. 13A to 13C.Alternatively, however, the cavity can also be produced in other ways,for example by means of a buried sacrificial layer.

In examples of the present disclosure, the smaller extent of therespective electrode in the direction of movement can be implemented byan insulating material that replaces a part of the respective electrode,by a cutout in the respective electrode, or by an oppositely dopedsemiconductor material in the respective electrode.

FIG. 10 shows one example of a capacitive sensor element in which thesmaller extent is implemented by an insulating material that replaces apart of the electrode. This electrode is also referred to hereinafter asa modified electrode, wherein this can be the movable electrode or thestationary electrode, depending on whether the first or the secondcapacitive sensor element of a micromechanical sensor is affected.

FIG. 10 shows a schematic sectional view of a capacitive sensor elementcomprising a substrate 100 and a cover 200. The cover 200 can be formedfor example by a dielectric layer, for example an oxide layer. The cover200 can completely enclose the top side of the substrate, for example inthe case of an acceleration sensor. In examples, the cover 200 can havean opening, for example in the case of a differential pressure sensor.

The substrate 100 has a buried cavity 102 and trenches 104, which definea movable electrode and a stationary electrode, for example as wasdescribed above with reference to FIGS. 6 to 9. One of the electrodesconstitutes a modified electrode 202, while the other electrodeconstitutes a non-modified electrode 204. Either the modified electrode202 or the non-modified electrode 204 can be implemented as a movableelectrode, while the other electrode is implemented as a stationaryelectrode. A cavity 206 can be provided in the cover 200, said cavityenabling a movement of the movable electrode vertically with respect tothe substrate plane in both directions. The substrate 100 can comprise ahighly doped contact layer 208. The modified electrode 202 and thenon-modified electrode 204 can be formed by a doped semiconductormaterial, wherein a part of the semiconductor material of the modifiedelectrode 202 is replaced by an insulator 210. Consequently, an extentof a sidewall surface 202 a of the modified electrode 202 situatedopposite a sidewall surface 204 a of the non-modified electrode 204 inthe direction of movement R2 is smaller than an extent of the sidewallsurface 204 a of the non-modified electrode 204. The insulator 210 canbe implemented by a dielectric, for example. The dielectric can compriseoxide or dioxide.

In the example shown in FIG. 10, an asymmetry is thus introduced by adielectric layer 210 being introduced into one of the electrodes on thetop side, said dielectric layer replacing the electrically conductivematerial of the electrode. As has been explained, this modification canconcern either the stationary electrode or the movable electrode. As hasbeen described with reference to FIGS. 2 and 3, a difference signal thatis different than 0 can be obtained as a result of the introduction ofsuch an asymmetry.

FIG. 11 shows one example of a capacitive element which differs from theexample shown in FIG. 10 with regard to the type of modification of themodified electrode 202. In the example shown in FIG. 11, the modifiedelectrode 202 has a counterdoping region 212 in order to bring aboutcorresponding asymmetrical characteristics. The counterdoping region hasa doping type that is different than the doping type of the substrate100. By way of example, said doping type can be a p-type doping if thesubstrate is n-doped. The counterdoping region can be produced forexample by means of an implantation into the surface of the substrate inthe region which defines the modified electrode. The counterdopingregion can be connected to a different electrical node than the rest ofthe modified electrode, such that it does not contribute to the activecapacitor area of the electrode. For this purpose, the counterdopingregion can have a contact region 212 a that is more highly doped than aremaining region 212 b of the counterdoping region. In examples, it ispossible for the counterdoping region 212 not to have a separateconnection, with the result that it is left floating and is isolatedfrom the remaining region of the modified electrode 202 via a pjunction.

FIG. 12 shows a further example of a capacitive sensor element in whichthe smaller extent of the modified electrode 202 in the direction ofmovement is implemented by means of a cutout in the modified electrode202. The cutout can be provided over the whole area on the electrode, asis indicated by a dashed line 214 in FIG. 12. Alternatively, the cutoutcan concern only a region of the modified electrode 202 that faces thenon-modified electrode 204. By way of example, the cutout can extendfrom the side facing the non-modified electrode 204 only as far as acentral region of the modified electrode 202, as indicated by a dashedline 216 in FIG. 12. The cutout can be produced for example by etchingback the substrate.

In examples of the present disclosure, the cavity 102 in the substrate100 can be produced by means of a so-called Venezia method. Said methodcan comprise etching trenches in a surface of the semiconductorsubstrate and annealing the semiconductor substrate in an H atmospherein order to cause the semiconductor material to flow back, in order tounite the trenches below the surface and to produce a buried cavity inthe semiconductor substrate. One example of such a method is explainedwith reference to FIGS. 13A to 13C. In one example of a correspondingmethod, a hard mask stack comprising an oxide layer 302 and apolysilicon layer 304 is deposited on a silicon substrate 300. The hardmask stack is structured and the structured hard mask stack is used toproduce trenches down to a depth t in the substrate 300. The depth t canbe 3 μm, for example. The resulting structure is shown in FIG. 13A.

The hard mask stack is subsequently removed, as is illustrated in FIG.13B. H2 annealing for silicon reflow is subsequently carried out. As aresult of the H2 annealing, the trenches 306 combine to form a buriedcavity 310 having a depth t₂. By way of example, the depth t₂ can be 1μm. Optionally, as is shown in FIG. 13C, an epitaxial silicon layer 308of suitable thickness and doping can be applied after the reflow. Afterthe H2 annealing, chemical mechanical polishing (CMP) can take place inorder to eliminate topology on the surface of the substrate on accountof the cavity formation. By means of this method a cavity 310 can beproduced in the semiconductor substrate 300, said cavity being closed bya doped semiconductor layer 312. In examples, the electrodes of thecapacitive sensor element or of the capacitive sensor elements can beproduced in the semiconductor layer 312, wherein the electrode wallsurface of one of the two electrodes of the capacitive sensor elementcan be modified in order to have a smaller extent in the seconddirection than the opposite electrode wall surface of the other of thetwo electrodes. In examples, modifying can comprise replacing a part ofthe semiconductor layer that faces away from the cavity by an insulatingmaterial, producing a cutout in a part of the semiconductor layer thatfaces away from the cavity, or producing a counterdoping in a part ofthe semiconductor layer that faces away from the cavity. In otherexamples, a modification could also take place in order to increase theextent of one of the two electrodes in the direction of movement.

As was explained above, it is possible to implement a silicon epitaxiallayer after the silicon reflow with the surface already closed. This canserve for increasing the mass of the movable part and thus for a greaterdeflection under acceleration in the case of an acceleration sensor. Inexamples, the reflow process can leave an approximately 1 μm thicksilicon layer above the cavity, the thickness of which layer can beincreased to 2 to 5 μm in examples by means of the epitaxy method.

In examples, as was explained above with reference to FIGS. 6 to 9, acontacting of the electrodes can be effected through an insulating layer110. One example of how such a contacting can be effected is describedwith reference to FIG. 14. FIG. 14 shows purely schematically forexplanatory purposes a substrate 400 having a buried cavity 402 andtrenches 404, which define two electrodes 406 and 408. Although notillustrated in FIG. 14, the electrodes 406 and 408 can be for examplecorresponding electrodes of a capacitive sensor element, one of whichelectrodes is modified and one is not. An insulating layer 410 isarranged on the substrate 400, wherein a highly doped contact layer 412can be arranged between the substrate 400 and the insulating layer 410.In order to produce contacts to the electrodes 406 and 408, trenches canbe produced in the insulating layer 410, said trenches extending as faras the highly doped layer 412. A liner layer 414 can be deposited in thetrenches, which liner layer can consist of Ti/TiN. By way of example,the liner layer 414 can be applied by sputtering. A heat treatment ofsaid layer 414 can subsequently be effected in order to form Tisilicide, which can have a lower contact resistance compared with a puremetal-semiconductor Schottky contact. Afterward, a conductive material416 can be applied or deposited in order to fill the trenches. Theconductive material can be a metal, such as e.g. tungsten. Afterward, awiring can be produced on the insulating layer 410, as is indicated bymetallization regions 418 in FIG. 14, in order to produce an electricalconnection to the electrodes.

Examples of the present disclosure provide methods for producing amicromechanical sensor element in which the electrode wall surface ofone of the two electrodes has a smaller extent in the second directionthan the opposite electrode wall surface. Examples of the presentdisclosure provide a method for producing a micromechanical sensorelement which has two capacitive sensor elements each having a first andsecond electrode, wherein, in the first capacitive sensor element, theelectrode wall surface of the movable electrode is produced with anextent in the second direction which is smaller than an extent of theopposite electrode wall surface of the stationary electrode, and whereinthe electrode wall surface of the stationary electrode of the secondsensor element is produced with an extent in the second direction whichis smaller than an extent of the opposite electrode wall surface of themovable electrode of the second sensor element in the second direction.In examples, producing the electrodes of the respective sensor elementcomprises producing a cavity in a semiconductor substrate, said cavitybeing closed by a doped semiconductor layer, wherein both electrodes ofthe respective sensor element are produced in the semiconductor layer,wherein the electrode wall surface of the electrode having the smallerextent in the second direction is modified in order to have the smallerextent in the second direction. In other examples, both electrodes ofthe respective sensor element can be produced in the semiconductorlayer, wherein the electrode wall surface of the electrode which doesnot have the smaller extent in the second direction is modified in orderto have a larger extent in the second direction than the oppositeelectrode wall surface of the other of the two electrodes.

Examples make it possible to produce corresponding micromechanicalsensors and sensor elements using microsystems technology (MEMStechnology).

One example of a method by which a capacitive sensor element having anon-modified electrode and a modified electrode, as is shown in FIG. 10,can be produced is described below with reference to FIGS. 15A to 15I.

FIG. 15A shows a semiconductor substrate 300, in which a buried cavity310 is produced. The semiconductor substrate 300 can consist of silicon,for example. The semiconductor substrate 300 can have a doping of morethan 5×10¹⁷. A semiconductor layer 312 is arranged above the cavity 310.The substrate shown in FIG. 15A can be produced for example by means ofa Venezia method as described above with reference to FIGS. 13A to 13C.A depth t₂ of the cavity 310 can be 1 μm, for example. A thickness t₃ ofthe semiconductor layer 312 can be 2 to 5 μm, for example. It should benoted at this juncture that the drawings are not true to scale in thisregard. The thickness of the semiconductor layer 312 can be increasedfor example by depositing an epitaxial silicon layer having acorresponding doping.

Proceeding from the structure shown in FIG. 15A, an oxide layer 314 isapplied to the surface of the semiconductor substrate 300, and a hardmask 316, which can consist of silicon nitride, for example, is appliedto said oxide layer. A shallow trench having a depth t₄, which can be 1μm, for example, is subsequently produced. The shallow trench is thenfilled with an oxide 318. Afterward, a CMP (chemical mechanicalpolishing) method can be carried out, which ends on the nitride hardmask 316. The resulting structure is shown in FIG. 15B. Oxideetching-back is subsequently carried out, which is known as “deglaze”,whereupon the nitride hard mask 318 is removed. A nitride etch stoplayer 320 is subsequently deposited. The latter can consist of SiO, forexample. The resulting structure is shown in FIG. 15C.

Proceeding from the structure shown in FIG. 15C, a further hard mask,which can be referred to as a trench hard mask, is applied to thenitride etch stop layer 320. The trench hard mask can comprise an oxidelayer 322 and a polysilicon layer 324. The oxide layer 322 can have athickness of approximately 2 μm, for example, and the polysilicon layer324 can have a thickness of approximately 500 nm, for example. Openings326 corresponding to the trenches to be produced in the semiconductorlayer 312 are then formed in the trench hard mask. The resultingstructure is shown in FIG. 15D. This is followed by nitride etching andoxide etching through the openings 326 as far as the surface of thesemiconductor substrate 300. The resulting structure is shown in FIG.15E. This is followed by silicon etching through the openings 326, as aresult of which etching trenches 104 are produced in the semiconductorlayer 312, said trenches defining the electrodes of the capacitivesensor element. The resulting structure is shown in FIG. 15F.

Proceeding from the structure shown in FIG. 15F, the hard maskcomprising the layers 320, 322 and 324 is removed. This is followed by acontact implantation with a high dose in order to produce the contactlayer 208. By way of example, for producing the contact implantation itis possible to use a phosphorus doping for an n-type silicon substrate.An activation annealing is subsequently carried out, which results inthe structure shown in FIG. 15G, in which the contact implantation isformed on the surface of the semiconductor substrate 300. As can be seenin FIG. 15G, the modified electrode 202 having the electrode wallsurface 202 a and the non-modified electrode 204 having the electrodewall surface 204 a are formed as a result.

One example of a method for producing a covering for the structure shownin FIG. 15G will now be described with reference to FIGS. 15H and 15I.For this purpose, firstly carbon is deposited onto the top side of thestructure shown in FIG. 15G and is structured in order to produce thecarbon layer 330 shown in FIG. 15H. Said carbon layer 330 constitutes asacrificial layer. Afterward, an oxide layer 200 is deposited andstructured in order to expose channels 332 serving for removing thecarbon layer 330. Afterward, the carbon layer 330 is removed and an HDPoxide cavity sealing (HDP=high density plasma) is carried out in orderto close the cavity 206 produced as a result of the removal of thecarbon layer. This results in the structure shown in FIG. 15I, whichcorresponds to the capacitive sensor element described above withreference to FIG. 10.

One example of a method for producing a capacitive sensor element suchas is shown in FIG. 11 is described below with reference to FIGS. 16A to16I.

FIG. 16A once again shows the starting substrate 300 and corresponds toFIG. 15A in this regard. Proceeding from this structure, firstly anoxide layer 314 is applied. A covering resist is applied to said oxidelayer 314 and is structured in order to produce therein a cutout 402having the form of a counterimplantation 404 to be produced in thesilicon layer 312. An implantation is subsequently carried out in orderto produce the counterimplantation 404 in the silicon layer 312. Thecounterimplantation 404 can be produced for example down to a depth of 1μm. The counterimplantation 404 has a doping type that is different thanthe doping type of the semiconductor substrate 300. The resultingstructure is shown in FIG. 16B.

Afterward, the resist 400 is removed and a nitride etch stop layer 320is deposited. Afterward, it is possible to carry out a masked high-dosecontact implantation for the semiconductor substrate 300 in order toproduce highly doped contact regions 406. Furthermore, it is optionallypossible to carry out a masked high-dose contact implantation for thecounterimplant 404 in order to produce a highly doped contact region 408in the counterimplant 404. The highly doped contact region 408 is of thesame doping type as the counterimplant 404. By way of example, thesemiconductor substrate 300 can be n-doped and the counterimplant 404can be p-doped. An annealing activation can subsequently be carried out.The resulting structure is shown in FIG. 16C.

A trench hard mask is then produced, such as was described above withreference to FIG. 15D. The resulting structure is shown in FIG. 16D.Afterward, the nitride etch stop layer 320 is opened through theopenings 326. Anisotropic oxide etching is then carried out selectivelywith respect to the semiconductor substrate 300. The resulting structureis shown in FIG. 16E.

Processing is subsequently carried out, which processing substantiallycorresponds to the processing described above with reference to FIGS.15F to 15I and thus need not be explained specifically in detail. Asshown in FIG. 16F, trench etching is carried out through the openings326 in order to produce the trenches 104 in the semiconductor layer 312.The hard mask is thereupon removed, as is shown in FIG. 16G, but acontact implantation is no longer carried out after the removal of thehard mask since the contact implantations have already been produced. Asis shown in FIGS. 16H and 16I, an encapsulation or a cover issubsequently produced, as was described above with reference to FIGS.15H and 15I. The resulting structure shown in FIG. 16I corresponds to acapacitive sensor element as shown in FIG. 11 and described above.

One example of a method for producing a capacitive sensor element suchas was described above with reference to FIG. 12 is explained below withreference to FIGS. 17A to 17C. In this case, the initial processingcorresponds to the processing described above with reference to FIGS.15A to 15G, and so reference is made to these figures in this regard.Proceeding from the structure shown in FIG. 15G, the oxide 318 and theoxide layer 314 are then removed. The structure shown in FIG. 17A isthereby obtained, in which a part of the modified electrode 202 isreplaced by a cutout 420. Proceeding from the structure shown in FIG.17A, an encapsulation or a cover can once again be produced using acarbon layer 330, as was described above with reference to FIGS. 15H and15I. This process is shown in FIGS. 17B and 17C, wherein, with regard tothe description of this process, reference is made to the aboveexplanations with regard to FIGS. 15H and 15I. The process describedwith reference to FIGS. 17A to 17C thus differs from the processdescribed with reference to FIGS. 15A to 15I in particular in that theisolation region 318 formed on the modified electrode is removed inorder to produce a cutout 420.

Only the steps relevant to the description of the present disclosurehave been described in each case with reference to FIGS. 15 to 17, andthe description of additional steps, such as the contacting of therespective electrodes, for example, has been omitted for the sake ofclarity.

Examples of micromechanical sensors in accordance with the presentdisclosure have two capacitive sensor elements, wherein the movableelectrode is correspondingly modified in a first of the sensor elements,and the stationary electrode is correspondingly modified in a secondcapacitive sensor element. FIG. 18A schematically shows a plan view of asecond capacitive sensor element 4 comprising a stationary electrode 16and a movable electrode 14, and FIG. 18B schematically shows a plan viewof a first capacitive sensor element 2 comprising a movable electrode 10and a stationary electrode 12. It should be noted at this juncture thatthe plan views in FIGS. 18A and 18B are once again purely schematic anddo not contain any details regarding the mounting of the individualelectrodes. The stationary electrode 16 is modified in the sensorelement 4 and the movable electrode is modified in the sensor element 2,as is indicated in each case by a rectangle 400. Contacts 402 for theelectrodes are furthermore schematically illustrated in FIGS. 18A and18B. Furthermore, the isolating trench 104 surrounding the electrodes isschematically illustrated in FIGS. 18A and 18B. In these figures, theouter rectangle 404 can indicate the extent of the buried cavity formedin the substrate. A rectangle 406 schematically shows the cavityproduced by the removal of the carbon layer during the production of theencapsulation. The dashed line 410 schematically indicates a sectionalplane for the sectional views in FIGS. 10 to 12.

The sensor elements 2 and 4 form one example of a micromechanical sensorin accordance with the present disclosure and can be arranged on acommon substrate or on separate substrates. In examples, the electrodesof the sensor elements can be formed in a common semiconductor layerarranged above a buried cavity, or in a semiconductor layer arrangedabove a plurality of cavities. Furthermore, the electrodes of thedifferent sensor elements can be arranged in different semiconductorlayers. In examples, the sensor elements can be monolithicallyintegrated and can be subjected to joint processing. In other examples,discrete processing of the sensor elements can take place. In examplesof the present disclosure, the first and second capacitive sensorelements can be formed in the same substrate. In examples, the first andsecond capacitive sensor elements can be formed in separate substrates,the substrate planes of which are parallel to one another.

In examples of the present disclosure, the first electrodes of the firstand second sensor element can be arranged on a common movable element. Aschematic plan view of one corresponding example is shown in FIG. 19.Reference signs identical to those in FIGS. 1A and 1B and FIGS. 18A and18B are used in FIG. 19. In accordance with the example shown in FIG.19, the first movable electrode 10 and the second movable electrode 14are arranged on a common movable element 500, which can be formed in asemiconductor layer arranged above a buried cavity 404. The element 500is suspended movably, as is indicated by a spring 502, such that themovable element is movable vertically with respect to the substrateplane. Furthermore, the micromechanical sensor shown in FIG. 19 has astationary electrode 12 and a stationary electrode 16. The stationaryelectrode 12 and the movable electrode 10 form a first capacitive sensorelement and the movable electrode 14 and the stationary electrode 16form a second capacitive sensor element. Corresponding electrode wallsurfaces 12 a and 10 a of the electrodes 10 and 12 are situated oppositeone another, and corresponding electrode wall surfaces 14 a and 16 a ofthe electrodes 14 and 16 are situated opposite one another, such that acapacitance is formed between them. As is indicated by rectangles 400 inFIG. 19, the movable electrode 10 and the stationary electrode 16 aremodified in this example.

In examples of the capacitive sensor elements, the mutually oppositeelectrode wall surfaces are planar. In other examples, the mutuallyopposite electrode wall surfaces are non-planar. FIGS. 20A and 20B showan example in which mutually opposite electrode wall surfaces arenon-planar, wherein FIG. 20A corresponds to FIG. 18A and FIG. 20Bcorresponds to an enlarged view of the region 520 in FIG. 20A in orderto illustrate mutually opposite electrode wall surfaces 14 b and 16 bthat are arranged interdigitally. It should be emphasized here that FIG.20B likewise shows a plan view, such that the movable electrode 16 ismovable into and out of the plane of the drawing, that is to sayvertically with respect to the substrate plane. In other examples, themutually opposite electrode wall surfaces can have other shapes.

In examples, the micromechanical sensor can be designed to detect anarbitrary physical variable that brings about a movement of the movableelectrodes in the second direction. In examples, the micromechanicalsensor can be designed as an acceleration sensor, wherein the firstelectrodes are movable in the second direction in response to anacceleration in the second direction. In such examples, the movableelectrode can be formed by an oscillating mass or can be arranged on anoscillating mass that is deflected in response to an acceleration. Inother examples, the micromechanical sensor can be designed as a pressuresensor, wherein the first electrodes are movable in response to apressure acting on the first electrode.

Examples of the present disclosure thus relate to a lateral capacitiveout-of-plane detection using asymmetrical electrodes. It is therebypossible to avoid ambiguities that can occur during a lateral capacitiveout-of-plane detection using symmetrical electrodes, which, around thepoint of rest of the movable electrode, have a substantially quadraticresponse of the capacitance to a deflection x. A modification of thestationary electrode in one sensor element and of the movable electrodein a second capacitive sensor element makes it possible to shift theextremum toward a negative and positive deflection, respectively, byapproximately the same absolute value a, C1=˜(x−a)², C2=˜(x+a)², as isshown in FIG. 3. It is thus possible to carry out an evaluation using asubtraction which makes it possible to achieve an increased linearityand which ideally leaves only a linear contribution, for example as theresulting output of a capacitive half-bridge or of a capacitivefull-bridge.

Examples of the disclosure relate to corresponding sensors and sensorelements which have mutually laterally opposite electrodes for acapacitive detection and are producible by means of a cost-effectivethin-film encapsulation. In examples, such a thin-film encapsulation canbe implemented using a sacrificial carbon and a dielectric layer, as wasdescribed above. In other examples, other types of encapsulation can beused, for example wafer bonding and the like. Examples can be producedusing monolithic microprocessing with a thin-film encapsulation. Otherexamples can be produced using a plurality of wafers, wherein forexample the electrodes of the capacitive sensor elements can be producedin a first wafer, while a second wafer and a third wafer can serve asupper and lower covering.

A capacitive detection such as is carried out in examples of the presentdisclosure can have, by comparison with piezoresistive detectionprinciples, lower thermal coefficients without a difficult resistancematching. In examples, the micromechanical sensor is a pressure sensor,for example a pressure sensor used in a tire pressure monitoring system.In examples, the micromechanical sensor is an acceleration sensor.Examples enable a virtually linear converter characteristic in order toenable signal extraction with high accuracy. Examples furthermore enablean acceleration detection in both a positive and a negative directionout of the securing plane (substrate plane) in order to enable securingboth in tire rubber and in a valve. Examples comprise an integration ofa corresponding micromechanical sensor with an in-plane accelerationdetection element. Examples of the present disclosure generally enable acapacitive out-of-plane deflection detection with a linearcharacteristic up to large deflections.

In the examples described above, the cavity is formed below theelectrodes using a silicon reflow technique (Venezia). However, thepresent disclosure is not restricted to such cavity formation. In otherexamples, the cavity can be implemented below the electrodes by means ofother techniques, for example using sacrificial layers composed of SiGeor oxide, or etching back from the rear side. In examples, the cavitycan also be formed by the bonding of a further substrate from the rearside.

In the examples described, the modification of the modified electrodewas achieved in each case by reducing the extent of the electrode wallsurface in the second direction. In other examples, the modification canbe achieved by adding electrode wall surface, wherein the non-modifiedelectrode then constitutes the electrode having the smaller extent inthe second direction. Such an enlargement of the extent can be achievedfor example by deposition of additional electrode material andstructured etching-back. Consequently, the present disclosure is notrestricted to such examples in which the smaller extent of one of theelectrodes is achieved by a subtractive modification of the electrodehaving the smaller extent, but rather also encompasses such examples inwhich an additive modification of the electrode having the larger extentis effected.

The present disclosure is not restricted to specific mountings inparticular of the movable electrode. In this respect, no specificdetails are indicated, rather all types of spring constructions can beused which yield a restoring force counteracting the deflection of themovable electrode from the rest position. In an area-efficient example,it is possible to use a seismic mass for an acceleration sensor or adeflectable membrane for a pressure sensor as movable electrode for bothcapacitive sensor elements. One such example was described above withreference to FIG. 19.

In examples of the present disclosure, the movable electrode itself canbe designed as a seismic mass. In other examples, the movable electrodecan be applied to a seismic mass.

Although evaluation circuits comprising a half-bridge circuit or afull-bridge circuit are described as examples, it is possible to useother evaluation circuits that form an output signal on the basis of thedifference between the capacitances of the two sensor elements.

In the examples described, the sacrificial layer for the thin-filmencapsulation is carbon. In other examples, other materials can be usedas sacrificial material, which materials can be structured andselectively removed in order to open materials, such as e.g. oxidematerial, SiN material or Si material. Examples make this possible usinga dry method in order to prevent sticking. In the examples described,the electrode material is a doped semiconductor material, in particulardoped silicon. In other examples it is possible to use other conductivematerials and suitable modifications of the mutually oppositeelectrodes. In other examples it is possible to use a nonconductivematerial with a suitable surface coating, once again with a suitablemodification of the mutually opposite electrodes. In examples, forproducing the cavity below the electrodes, an SOI substrate with localelimination of the buried oxide layer can be used instead of the methoddescribed. In other examples, it is possible to use an Si—SiGe—Sisubstrate with local SiGe removal, as is known for example fromso-called “Silicon-on-Nothing” elements.

Although some aspects of the present disclosure have been described asfeatures in association with a device, it is clear that such adescription can likewise be regarded as a description of correspondingmethod features. Although some aspects have been described as featuresin association with a method, it is clear that such a description canalso be regarded as a description of corresponding features of a deviceor of the functionality of a device.

In the detailed description above, in some instances various features inexamples have been grouped together in order to rationalize thedisclosure. This type of disclosure ought not be interpreted as theintention that the claimed examples have more features than areexpressly indicated in each claim. Rather, as expressed by the followingclaims, the subject matter can reside in fewer than all features of anindividual example disclosed. Consequently, the claims which follow arehereby incorporated in the detailed description, and each claim canstand as a distinct separate example. While each claim can stand as adistinct separate example, it should be noted that, although dependentclaims in the claims refer back to a specific combination with one ormore other claims, other examples also encompass a combination ofdependent claims with the subject matter of any other dependent claim ora combination of each feature with other dependent or independentclaims. Such combinations shall be encompassed, unless it is explainedthat a specific combination is not intended. Furthermore, it is intendedthat a combination of features of a claim with any other independentclaim is also encompassed, even if this claim is not directly dependenton the independent claim.

The examples described above are merely illustrative of the principlesof the present disclosure. It should be understood that modificationsand variations of the arrangements and of the details described areevident to those skilled in the art. It is therefore intended that thedisclosure is limited only by the appended patent claims and not by thespecific details set out for the purpose of describing and explainingthe examples.

What is claimed is:
 1. A method for producing a micromechanical sensorelement having a first electrode and a second electrode, whereinelectrode wall surfaces of the first and the second electrodes aresituated opposite one another in a first direction and form acapacitance, wherein a first one of the first electrode and the secondelectrode is movable in a second direction, which is different than thefirst direction, in response to a variable to be detected, and a secondone of the first electrode and the second electrode is fixed, the methodcomprising: producing a cavity in a semiconductor substrate, the cavitybeing closed by a doped semiconductor layer; and producing the first andthe second electrodes in the doped semiconductor layer, includingmodifying the electrode wall surface of the first electrode in order tohave a smaller extent in the second direction than the electrode wallsurface of the second electrode, wherein modifying the electrode wallsurface of the first electrode comprises forming a counterdoped regionin the electrode wall surface of the first electrode in a part of thesemiconductor layer which faces away from the cavity, and wherein thecounterdoped region is adjacent to a remaining portion of the electrodewall surface of the first electrode.
 2. The method as claimed in claim1, wherein: the counterdoped region comprises a first doping type thatis different than a second doping type of the semiconductor layer, andthe counterdoped region does not contribute to an active capacitor areaof the first electrode.